It has "been known for some seven decades that the presence of a free surface, onto which condensation may take place, is required before the phase change from vapour to liquid will proceed, and that the smaller is the radius of curvature of the surface, the higher the degree of super saturation of the vapour must "be to induce the phase change. Such surfaces were found to be present in any normal sample of atmospheric air in the form of condensation nuclei, ranging in size and in effectiveness as centres at which condensation would take place from relatively large dust particles, effective at very small degrees of supersaturation of less than 1%t to minute clusters of vapour molecules, stabilized by the presence of a charge, requiring a supersaturation greater than 400% to be effective.